Described herein is a process for improving the color of thermoplastic polymers by contacting the monomer(s) before or during polymerization or the polymer after its formation with an alkali or alkaline earth metal formate.
Thermoplastic polymers such as a polyarylate, poly(etherimide), polyester, aromatic polycarbonate, poly(aryl ether), etc., are high temperature, high performance thermoplastic polymers with a good combination of thermal and mechanical properties. However, many of the processes and/or raw materials used to produce these polymers can result in the production of a highly colored polymer. For example, polyarylates produced by the diacetate process have a tendency to be colored. Such colored polymers could be unsuitable for use in some end use applications.
Carboxylic acid salts have been used to improve the thermal stability and crazing resistance of aromatic copolyesters as described in U.S. Pat. No. 4,229,332. The polyester in said patent is prepared by an acid chloride process, and it is highly probable that these carboxylic acid salts, such as sodium acetate neutralize the residual hydrochloric acid or acid end groups to improve the thermal stability and crazing resistance of the polyester in boiling water. Such carboxylic acid salts have also been used to prevent the discoloration of polyacetal by scavenging the strong acid catalysts. (Chem. Abs. 82:59097h, Japan Kokai No. 74,99,339).
Therefore, it was surprising that alkali or alkaline earth metal formates, such as sodium or potassium formate when used as an additive in the polymerization of thermoplastic polymers, particularly polyarylates produced by the diacetate process in the absence of a strong acid catalyst, lowered the color of the corresponding polymer significantly. However, this is not a process where all carboxylic acid salts have a beneficial effect on the color of the polymer. For example, sodium acetate does not reduce the color of the polymers, particularly polyarylates produced by the diacetate process. Therefore, the sodium formate appears to bleach polyarylates by a mechanism different from the typical acid neutralization process which prevents the decomposition and degradation reactions in other systems. Potassium formate also bleaches the polyarylate, but zinc formate results in a darker polyarylate than one with no additive. The utility of the metal formate seems to depend on its thermal and oxidative stability properties.